Apparatus for melting titanium

ABSTRACT

Apparatus for melting and casting a titanium alloy including an inductively heated furnace surrounding a refractory crucible, the titanium alloy to be melted being suspended as a slug in spaced relation to the walls of the crucible. A titanium alloy disc is positioned in the crucible at the discharge outlet thereof so that when the temperature of the slug is brought to the melting point, the resulting molten metal melts through the disc and is then directed immediately to a mold.

United States Patent Clark 51 May 23, 1972 [54] APPARATUS FOR MELTING TITANIUM 2,826,624 3/ 1958 Reese et al 13/27 [72] Inventor: Clyde C. Clark, Euclid, Ohio Primary Examiner Bemard A Gilheany [73] Assignee: TRW Inc Cleveland, Ohio Assistant Examiner-R. N. Envall, Jr. 7 Filed: Mar. 1971 Attorney-Hill, Sherman, Meroni, Gross & Simpson [2]] Appl. No.: 122,853 [5 7] ABSTRACT Ra g d Us, A li ti D t Apparatus for melting and casting a titanium alloy including an inductively heated furnace surrounding a refractory cruci- [62] DlVlSlOll of Ser. No. 767,168, Oct. 14, 1968, Pat. No. ble the titanium alloy to be melted being Suspended as a slug l in spaced relation to the walls of the crucible. A titanium alloy disc is positioned in the crucible at the discharge outlet [52] US. Cl ..l3/26, 164/337 thereof so that when the temperature of the g is brought to [51] Int. Cl. ..F27d 3/15, HOSb 5/00 the melting point the resulting molten metal melts ough l; 5 0 {58] Field of Search 13/26 27 5 164/337 7 ,1 the disc and is then directed immediately to a mold. [561 Rem-mm 6 Claim, 3 Drawing Figures UNITED STATES PATENTS 2,754,347 7/1956 Wroughton et al. 13/27 X 25- 2;? 1 f L ,7 x T APPARATUS FOR MELTING TITANIUM CROSS REFERENCE TO RELATED APPLICATION This application is a division of my copending application, Ser. No. 767,168 filed Oct. 14, 1968 now US. Pat. No. 3,598,168.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of melting and casting apparatus for melting titanium alloys at a titanium content of 70 percent and above, including commercially pure titanium which contains 99.8 percent Ti. The invention is directed to a furnace assembly including an inductively heated, electrically conductive crucible having a discharge opening therein covered by a titanium alloy disc. The titanium alloy to be melted is in the form of a slug which is supported in spaced relation to the walls of the crucible, but is entirely confined therein. Theslug of the titanium alloy is heated to near its melting temperature while suspended out of contact with the walls of the crucible, and then additional heat is applied to the slug to melt the same. The molten metal is then superheated while it is discposed on the titanium alloy disc which preferably has the same chemical composition as the material of the slug. Continued application of heat causes the molten metal to melt through the titanium alloy disc, and the molten titanium alloy can then be introduced directly into a mold.

2. Description of the Prior Art Titanium alloys used in forming shaped castings are usually skull melted. in water-cooled, crucible arc melting units. The temperature control in this type of melting is difficult because the molten titanium is cooled rapidly by the water-cooled crucible when power from the arc is terminated. A resulting variation in pouring temperature may cause many casting defects. If the temperature is too high, dimensional and metallurgical problems result. If the pouring temperature is too low, the castings misrun and also contain metallurgical defects.

One of the most serious problems in malting and pouring titanium is the avoidance of contamination. Titanium has a strong affinity for hydrogen, nitrogen, oxygen and carbon. Excess absorbed hydrogen is removed from titanium alloys by expensive processing such as vacuum annealing. According to current specifications, the hydrogen content of titanium should be limited to about 125 to 200 parts per million. Above these limits, hydrogen embrittles most titanium alloys, and reduces their impact strength and notch tensile strength, causing brittle failure under sustained loads at low stresses.

Other contaminants cause titanium alloys to become embrittled. This embrittlement is usually manifested by the formation of a brittle surface layer which must be removed by pickling, grinding or machining.

The drawbacks to commercial skull melting procedures as presently practiced in the casting of titanium are further described in the article entitled A Look at Worldwide Titanium Technology appearing in the September 1968 issue of Metal Progress, pages 60 to 71.

US. Pat. No. 3,435,878 to Howard et al. describes a method of casting metals by means of a refractory, electrically non-conductive melting chamber provided with a discharge port in the floor thereof in communication with the inlet port of a mold. The outlet port is closed with a solid metal surface having a melting point not greater than the pouring temperature. The charge of metal is inductively heated in a non-conductive chamber until the charge becomes molten and melts through the solid metal surface. The use of the non-conducting crucible permits the charge to become directly inductively coupled to the induction coil. Because of this direct inductive coupling, energization of the coil not only generates heat in the charge to cause melting of the charge, but is said to produce a vigorous electromagnetic stirring action which tends to keep the molten metal at a constant temperature.

SUMMARY OF THE INVENTION The present invention relates to an apparatus for melting and pouring titanium alloys in which a slug of the titanium alloy to be melted is first suspended within an electrically conductive crucible to be fully enclosed thereby, the slug being out of contact with the walls of the crucible. The crucible is preferably made of dense graphite and has an aperture in the base thereof. The aperture is covered by means of a disc of titanium alloy having the same chemical analysis as the slug to be melted, and in any event, being devoid of any contaminating elements which would affect the physical properties of the titanium melt. The slug of titanium alloy is inductively heated while so suspended to a temperature just below the melting point of the slug. When the entire slug is substantially at this temperature, an additional amount of heat is provided by the induction heating apparatus to raise the temperature of the slug above the melting temperature, whereupon a molten pool of titanium alloy is formed on the titanium alloy disc. The molten alloy is superheated and, in a short period of time, it melts through the titanium alloy disc and runs out of the crucible through the aperture. The time of contact of the molten titanium alloy with the refractory crucible is thus held to a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is applicable to the melting and pouring of titanium alloys generally, where contamination is a problem. It is applicable to the melting of commercially pure titanium (99.2 percent Ti) as well as to alpha titanium alloys (Ti-5Al2.5Sn) to alpha-beta titanium alloys (Ti-2Fe-2Cr-2 Mo) (Ti-8Mn) (Ti-4Al-Mn) (Ti-4Al-3Mo-1V) (Ti-5Al l.5Fe)DCr-l.2Mo) (Ti-6Al-4V) (Ti7Al-4Mo) and to beta titanium alloys (Ti-3All 3Vl lCr).

Turning now to the drawings, in FIG. 1 reference numeral 10 indicates generally a melting and pouring apparatus which can be used for the purposes of the present invention. The apparatus 10 includes a graphite crucible 11 having a neck 12 providing a discharge opening 13. The graphite crucible 11 is positioned on a ceramic support plate 14 having an aperture 15 therein accommodating the neck 12 of the crucible. A cylindrical ceramic heat shield 16 is positioned about the graphite crucible 11 in coaxial relation. Surrounding the heat shield 16 is a high frequency induction coil 17 connected to a suitable source of power (not shown). The coil 17 rests on one or more positioning blocks 18 standing on the ceramic support plate 14.

'Overlying the discharge opening 13 is a disc 19 of titanium alloy, the disc 19 having a reduced thickness central portion 20. A slug 21 of the titanium alloy to be melted is supported in spaced relation to the walls of the crucible 11 as well as the disc 19 by a support rod 22 which is suitably fastened to the slug 21 as by means of a threaded engagement. A thermocouple 23 is positioned centrally of the slug 21 to give an indication of the temperature at that portion of the slug.

When the slug is supported as illustrated in FIG. 1 of the drawings, sufiicient power is applied to the coil 17 to preheat the entire suspended slug to a temperature just below its melting point. The melting points of commercial titanium alloys range from about 2,800 to 3,135 F. for the commercially pure titanium. With induction heating, the heat buildup occurs on theouter surface adjacent to the induction coil 17 and progresses inwardly, mainly by conduction. The progress of the heating is monitored by the thermocouple 23. When the slug is unifirmly heated to a temperature within say 100 F. of its melting point, full power is applied to the coil 17, and melting is completed in a very short time. The molten metal 24, as illustrated in FIG. 2 of the drawings, is collected momentarily by the titanium alloy disc 19. This permits the melt to reach the desired superheat of 100 to 400 F. or so. The superheat can be controlled by the thickness of the reduced portion 20 of the disc and its location with respect to the coil 17. The molten bath 24 rapidly melts the disc 19 as illustrated in FIG. 3, and proceeds through the discharge opening 13 into the gate 25 of a mold assembly for producing castings.

lt should be understood that the melting and pouring is done under non-contaminating conditions such as under vacuum conditions or in the presence of an inert gas such as argon. Since such melting and pouring conditions are customary for titanium alloys, the furnace enclosures have not been illustrated in the drawings.

It has been found that through the use of the apparatus of the present invention, where the slug is preheated without crucible contact, that a much shorter molten cycle results, and considerably less carbon contamination is encountered.

I claim as my invention:

1. An apparatus for melting titanium alloys which comprises a refractory, electrically conductive crucible, induction heating means positioned to heat the interior of said crucible, support means extending into said crucible, a slug of titaniumalloy secured to said support means and confined completely within said crucible in spaced relation to the walls thereof, said crucible having a discharge opening therein, and a titanium alloy disc covering said opening.

2. An apparatus as recited in claim 1 including a temperature sensing means positioned to sense the temperature centrally of said slug while said slug is suspended in said crucible.

3. An apparatus as recited in claim 1 which includes a ceramic heat shield positioned between said crucible and said induction heating means.

4. An apparatus as recited in claim 1 in which said disc is composed of the same titanium alloy as said slug.

5. An apparatus as recited in claim 1 in which said refractory crucible is composed of graphite.

6. The apparatus of claim 1 which includes a mold positioned directly below said discharge opening to receive molten metal issuing therefrom. 

1. An apparatus for melting titanium alloys which comprises a refractory, electrically conductive crucible, induction heating means positioned to heat the interior of said crucible, support means extending into said crucible, a slug of titanium alloy secured to said support means and confined completely within said crucible in spaced relation to the walls thereof, said crucible having a discharge opening therein, and a titanium alloy disc covering said opening.
 2. An apparatus as recited in claim 1 including a temperature sensing means positioned to sense the temperature centrally of said slug while said slug is suspended in said crucible.
 3. An apparatus as recited in claim 1 which includes a ceramic heat shield positioned between said crucible and said induction heating means.
 4. An apparatus as recited in claim 1 in which said disc is composed of the same titanium alloy as said slug.
 5. An apparatus as recited in claim 1 in which said refractory crucible is composed of graphite.
 6. The apparatus of claim 1 which includes a mold positioned directly below said discharge opening to receive molten metal issuing therefrom. 